Merve Ergun Dönmez: Rhodium Catalyzed Fullerene C60 Hydroarylation with Arylboronic Acids: Development and Mechanistic Study Using HPLC Analysis and NMR Spectroscopy

Abstract

The unique spherical carbon molecule fullerene C60 holds potential across several application areas, particularly after modification and fine-tuning to achieve desired properties. Due to its accessibility and broad scope, the functionalization of fullerene C60 via rhodium-catalyzed hydroarylation using organoboron compounds has significant potential for being an efficient tool for fullerene and alkene/alkyne cross-coupling. Here, we explored this reaction in an attempt to understand and take advantage of its possibilities fully. 

A straightforward and simple analytical method was developed employing HPLC using commonly available stationary phases to separate and isolate reaction components of hydroarylation of C60 using arylboronic acids. This method was validated for quantifying unreacted C60 in a hydroarylation reaction and used to determine its conversion. Combining this methodology with 1H and 19F NMR quantification methods using internal standards provided fast and simple screening of the reaction components. 

Using these tools, the hydroarylation reaction conditions were improved by probing changes in the rhodium catalyst load, time, reaction temperature, and the effect of different protic additives. The arylboronic acid scope for hydroarylation of fullerene C60 was investigated using these optimized conditions. This was followed by a screening of hydroarylation reactions for selected alkenes and alkynes using fluorinated arylboronic acid with analysis via 19F NMR spectroscopy. The formation rate of products and consumption rate for arylboronic acids in the hydroarylation reaction of C60 were compared when different protic solvents were added or when protected arylboronic acids were used. 

A deeper investigation of the possible reaction mechanism required individual assessment of the components. Depending on the nature of the protic solvent added to the reaction mixture, arylboronic acids were found to exhibit various forms in different ratios. A deuterium labeling study was conducted to provide evidence of the source of the proton in the hydroarylation product. It was concluded that the 1,4-rhodium shift might take place when electron-poor arylboronic acids were used in hydroarylation. 

Lastly, a stable intermediate arene η6 Rh complex forms when electron-rich arylboronic acids are used in the hydroarylation of fullerene C60. The formation of such a complex was proved by the significant change in 1H and 13C NMR chemical shifts of aryl signals, which showed specific arene 103Rh-13C couplings in the 13C NMR spectrum.